High Bandwidth Demodulator System And Method

ABSTRACT

A method of analysing an input signal, the method including the steps of: (a) dividing a first input signal into first and second orthogonal signal polarisation components; (b) dividing a second input signal into orthogonal first and second orthogonal local polarisation components; (c) mixing the first orthogonal signal component with the second orthogonal local polarisation component to provide a first mixed signal; (d) mixing the second orthogonal signal component with the first orthogonal local polarisation component to provide a second mixed signal; (e) analysing the first and second mixed signal to determine the polarisation or phase information in the input signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. Pat. No. 12/842,333,filed on Jul. 23, 2010, entitled “High Bandwidth Demodulator System andMethod” which claims priority to U.S. Provisional Patent ApplicationSer. No. 61/322,076, filed on Apr. 8, 2010, entitled “High BandwidthDemodulator System and Method” and to U.S. Provisional PatentApplication Ser. No. 61/228,940, filed on Jul. 27, 2009, entitled “HighBandwidth Demodulator System and Method,” the entire disclosures ofthese patent applications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the analysis of optical signals ingeneral. In one application the invention relates to high capacityoptical demodulation and, in particular, discloses a high capacityoptical system and method for demodulation of the electric field vector(phase and amplitude) for a given pair of polarization states.

BACKGROUND OF THE INVENTION

Demand for the accurate analysis of optical signals continues to be ofsignificant importance in many fields. For example, in accuratemeasurement, it is often important to be able to accurately sample anoptical signal. The need for accurate measurement is prevalent in a widerange of fields, including optical telecommunications and opticalmeasurement.

For example, demand for high capacity data transmission continues togrow and grow. One dominant form of transmission is optical transmissionover optical fibers or the like or optical free space transmission.Current planning demands future 100 gigabytes per second (100G) systems.With such high capacity transmission systems, there is a need todemodulate a received optical signal.

One suitable encoding methodology for high bandwidth opticaltransmission is Differential Quaternary Phase-Shift Keying (DQPSK). Insuch a system, information is encoded in the phase of the transmittedsignal. In particular, encoding is provided in phase changes in thetransmitted signal.

One high capacity DQPSK transmission system for optical communicationsis Dual Polarization with Quadrature Phase Shift Keying. Example DP-QPSKsystems are set out in the following:

-   -   U.S. Pat. No. 5,473,463 to Van Deventer discloses an optical        receiver known as an optical hybrid device;    -   U.S. Patent application publication number 2007/0223932 to Hsich        also discloses a coherent optical receiver device.    -   “Recent advances in coherent optical communication” by Guifang        Li, Advances in Optics and Photonics 1, 279-307 (2009) discusses        the principals of coherent optical receivers.    -   Other optical hybrid devices are discloses in U.S. Pat. No.        7,209,670 to Fludgerfl, U.S. Pat. No. 7,315,575 to Sunfl, U.S.        Pat. No. 6,917,031 to Sunfl.

Normally, in each of the above referenced designs, it is common toimplement detection coherently by means of mixing of the electric fieldvectors of the aligned polarization states of an input signal and alocal oscillator. A number of problems are provided with implementationof such designs. The aforementioned arrangements often rely uponinterferometric structures with one arm of the interferometer providinga 90 degree phase delay. Unfortunately, the requirement for a 90 degreephase delay can often lead to difficulty in meeting tolerances. A 90degree phase shift is equivalent to, at standard optical transmissionbands, to the utilization of a 400 nanometre optical element. Allowingfor say a 2 to 3 degree tolerance accuracy places about a 10 nanometertolerance accuracy on the phased delayed element. This is difficult toprovide, especially where temperature variations occur. Further, theinterferometric system often leads to extremely tight tolerances onalignment. This normally leads to a high expense in manufacturingoptical hybrid devices or the additional complexity of actively tuningthe phase delay based upon a feedback from the signal.

As phase and amplitude detection techniques are highly likely to beutilized in 100 gigabit transmission systems (100G), there is a generalneed for an effective form of polarization processing of transmittedsignals so as to provide for electric field phase and amplitudedetection and decoding. The utilization of polarization multiplexedphase encoding in 100G coherent systems allows for higher data rates oftransmission. Detection of the electric field vector of the opticallytransmitted signal is particularly advantageous as it permits thecalculation and mitigation of many transmission impairments anddistortions such as chromatic dispersion induced pulse spreading andpolarization mode dispersion.

The coherent transmission system is likely to rely on a dualpolarization with a quadrature phase shift key modulation scheme(DP-QPSK). This is known to be especially efficient and providesimproved signal to noise and allows for utilization of CMOS electronicdecoder systems. Other forms of optical encoding are known and are alsoapplicable with the present invention.

Turning initially to FIG. 1, there is illustrated one form of referencedesign 10 for a DP-QPSK transmitter. The reference design illustratestransmission on one wavelength band only. It will be obvious to thoseskilled in the art that it can be combined via multiplexing with othertransmitters for other wavelength bands. In the DP-QPSK transmissionsystem, an input laser 11 of predefined frequency and polarization stateis input 12 and is interconnected to a number of modulators 13-16 whichact under electronic control of drivers 17-20. The modulators 13-16 actto phase modulate the signal in a known controlled manner.

The modulators act to phase encode an input data stream. Polarizationmultiplexing is then provided by polarization rotation element 21 whichoutputs an orthogonal polarization 23 to second polarization 22. The twoorthogonal polarizations are combined 25 by beam combiner fortransmission.

The signal is then transmitted in a particular wave length band. Duringtransmission, the orthogonality of the polarization states issubstantially maintained although rotation of the overall polarizationstate may occur.

The receiver is then responsible for decoding the transmitted signal soas to extract the data information that has been encoded by thetransmitter.

SUMMARY OF THE INVENTION

It is an object of the present invention provide for an improved form ofanalysis of optical signals, particularly phase encoded or polarizationmultiplexed optical signals.

In accordance with a first aspect of the present invention, there isprovided a method of analysing an input signal, the method including thesteps of: (a) dividing a first input signal into first and secondorthogonal signal polarisation components; (b) dividing a second inputsignal into orthogonal first and second orthogonal local polarisationcomponents; (c) mixing the first orthogonal signal component with thesecond orthogonal local polarisation component to provide a first mixedsignal; (d) mixing the second orthogonal signal component with the firstorthogonal local polarisation component to provide a second mixedsignal; (e) analysing the first and second mixed signal to determine thepolarisation and/or phase information in the input signal.

In some embodiments, the second input signal can comprise a time delayedversion of the first input signal. In some embodiments, the input signalpreferably can include information encoded in a periodic signal and thetime delay can comprise substantially one signal period. In someembodiments, the encoding can be DP-QPSK encoding. In other embodiments,the second input signal can comprise a local oscillatory signal having apredetermined polarisation.

In accordance with a further aspect of the present invention, there isprovided a method of analysing an input signal, the method including thesteps of: (a) dividing the input signal into first and second orthogonalsignal polarisation components; (b) dividing a local oscillatory signalhaving a predetermined polarisation into orthogonal first and secondorthogonal local polarisation components; (c) mixing the firstorthogonal signal component with the second orthogonal localpolarisation component to provide a first mixed signal; (d) mixing thesecond orthogonal signal component with the first orthogonal localpolarisation component to provide a second mixed signal; (e) analysingthe first and second mixed signal to determine the polarization and/orphase information in the input signal.

The step (e) further preferably can include the steps of: (e1) splittingthe power of the first mixed signal into at least a first and secondmixed sub-signals; (e2) delaying one orthogonal polarisation componentof the first sub-signal relative to the second component by π/2 radiansto produce a phase delayed first sub-signal; (e3) dividing the phasedelayed first sub signal into orthogonal components and the second mixedsub-signal into orthogonal components.

The step (e) further preferably can include the steps of: (e1) delayingone orthogonal polarisation component of the first sub-signal relativeto the second component by π/2 radians to produce a phase delayed firstsub-signal; (e2) dividing the phase delayed first sub signal intoorthogonal components and the second mixed sub-signal into orthogonalcomponents.

In accordance with a further aspect of the present invention, there isprovided an apparatus for analysis of the polarisation state of an inputsignal, the apparatus including: a polarisation diversity elementinterconnected to the input signal and local oscillating signal forseparating orthogonal components of the input signal and a predeterminedsecond optical signal into first and second orthogonal signalcomponents; a first polarisation splitting element for further splittingthe first and second orthogonal signal components into furtherorthogonal subcomponents, and first and second orthogonal second opticalsignal components into further orthogonal subcomponents; a polarisationelement for polarisation aligning predetermined ones of thesub-components; a polarisation translation element for spatiallyaligning groups of polarisation aligned sub-components to produce aseries of spatially aligned subcomponents; a second polarisationsplitting element for splitting the aligned subcomponents into outputcomponents.

In accordance with a further aspect of the present invention, there isprovided an apparatus for decoding polarization encoded input signals,the apparatus including: a local oscillator outputting a localoscillating signal having a predetermined polarization state; an inputfor inputting the polarization encoded input signal; a polarisationdiversity element interconnected to the input signal and localoscillating signal for separating orthogonal components of each intofirst and second orthogonal signal polarisation components and the localoscillating signal into first and second orthogonal local signalcomponents; a first polarisation splitting element for further splittingthe first and second orthogonal signal components into furtherorthogonal subcomponents, and first and second orthogonal local signalcomponents into further orthogonal subcomponents; a polarisation elementfor polarisation aligning predetermined ones of the sub-components; apolarisation translation element for spatially aligning groups ofpolarisation aligned sub-components to produce a series of spatiallyaligned subcomponents; a second polarisation splitting element forsplitting the aligned subcomponents into output components.

In accordance with a further aspect of the present invention, there isprovided an apparatus for analysis polarization and/or phase encodedinput signals, the apparatus including: an input for inputting thepolarization encoded input signal; a delay element for producing adelayed version of the input signal; a polarisation diversity elementinterconnected to the input signal and the delayed version of the inputsignal for separating orthogonal components of each into first andsecond orthogonal signal polarisation components and the delayed versionof the input signal into first and second orthogonal delayed signalcomponents; a first polarisation splitting element for further splittingthe first and second orthogonal signal components into furtherorthogonal subcomponents, and first and second orthogonal delayed signalcomponents into further orthogonal subcomponents; a polarisation elementfor polarisation aligning predetermined ones of the sub-components; apolarisation translation or deflection element for spatially aligninggroups of orthogonal polarisation aligned sub-components to produce aseries of spatially aligned subcomponents; a second polarisationsplitting element for splitting the aligned subcomponents into outputcomponents.

In accordance with a further aspect of the present invention, there isprovided a method of measuring polarization and/or phase informationpresent in an input signal, the method including the steps of: (a)dividing a first input signal into first and second orthogonal signalpolarisation components; (b) dividing a second input signal intoorthogonal first and second orthogonal second signal polarisationcomponents; (c) mixing the first input signal with the first and secondorthogonal second signal polarisation components to provide first andsecond mixed signals; and (d) analysing the first and second mixedsignal to determine the polarisation information in the input signal.

In accordance with a further aspect of the present invention, there isprovided a method of measuring polarization information present in aninput signal, the method including the steps of: (a) dividing the inputsignal into first and second orthogonal signal polarisation components;(b) dividing a local oscillatory signal having a predeterminedpolarisation into orthogonal first and second orthogonal localpolarisation components; (c) mixing the input signal with the firstlocal polarisation component and the second orthogonal localpolarisation component to provide first and second mixed signals; and(d) analysing the first and second mixed signal to determine thepolarisation information in the input signal.

In accordance with a further aspect of the present invention, there isprovided a method of measuring polarization information present in aninput signal, the method including the steps of: (a) dividing a localoscillatory signal having a predetermined polarisation into orthogonalfirst and second orthogonal local polarisation components; (c) mixingthe input signal with the first local polarisation component and thesecond orthogonal local polarisation component to provide first andsecond mixed signals; and (d) analysing the first and second mixedsignal to determine the polarisation information in the input signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Benefits and advantages of the present invention will become apparent tothose skilled in the art to which this invention relates from thesubsequent description of exemplary embodiments and the appended claims,taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates schematically a standard proposed transmitterstructure for DP-QPSK networks;

FIG. 2 illustrates schematically a DP-QPSK receiver plan of a firstembodiment;

FIG. 3 illustrates schematically the polarization demodulation principleutilised in the first embodiment;

FIG. 4 illustrates an alternative polarization demodulation operation;

FIG. 5 and FIG. 6 illustrate example binary phase demodulations suitablefor use with the first embodiment;

FIG. 7 illustrates one form of implementation of a receiver of the firstembodiment;

FIG. 8 illustrates the polarization state transitions for inputtedsignals in the arrangement of FIG. 7;

FIG. 9 illustrates a prototyping of a receiver of the first embodiment;

FIG. 10 illustrates schematically the operation of an alternativeembodiment;

FIG. 11 illustrates one form of optical delay line; and

FIG. 12 illustrates an alternative prototype receiver.

DESCRIPTION OF THE PREFERRED AND OTHER EMBODIMENTS

The preferred embodiments provide a method and apparatus for phaseanalysis of polarization multiplexed phase modulated signals. Forexample polarization independent detection and demodulation of a singlepolarization QPSK format can be achieved. The requirement for activepolarization control and the demodulator could be used further fordemodulation of single or polarization multiplexed optical OFDM(Orthogonal Frequency Division Multiplexed) systems. Alternatively, thepreferred embodiment could be used in the general analysis of phaseinformation in signals.

Turning to FIG. 2, there is illustrated schematically the generalstructure of a first embodiment optical processing unit 30. In this unit30, the inputs include a local oscillator 36 having a 45 degreepolarization state in addition to the input signal 37. The signal 37 ismade up of orthogonal polarizations with each of the orthogonalpolarizations further having a phased encoding in accordance with theDP-QPSK standard.

The first embodiment comprises three stages. The first stage 31separates orthogonal components of the input local oscillator 36 and theinput signal 37. This is achieved by utilizing a polarization splittingcrystal 40, 41. Each of the polarization splitters split thepolarization into vertical and horizontal components. The next stage 32analyses each of the orthogonal polarizations by means of a polarizationmixer. The polarization mixer outputs 33 can either be fiber coupled tooutputs or coupled to a PIN-TIA array of detectors at a small pitch. Inthe arrangement 30, the polarization states upon input to the mixerstage 32 are orthogonal. The polarization mixer relies on combiningorthogonal polarization states of signal and local oscillator and thenanalyzing the generated polarization state to generate the requiredsignals.

Turning now to FIG. 3, there is illustrated an example of the operationof each of the polarization mixers 32 of FIG. 2. The first polarizationstage takes orthogonal polarization inputs 51 and combines them 52. Thenthey are split 53 with the power split into upper and lower channels. Inone embodiment, the splitter could be a transmission grating withperiodic structure designed to provide an angular separation betweenpositive and negative orders of the grating or could be provided by apartial mirror.

One of those channels undergoes a polarization retardation 54 by meansof quarter wave plate 56. Next polarization splitter 57 splits thepolarization states into diagonal orthogonal components to provideoutputs 59.

An alternative arrangement of the polarization mixer is illustrated 70in FIG. 4. In this arrangement, the power splitting process is movedbefore the polarization combination. The input polarizations 71 aresubject to a power splitting 72 via power splitter 73. Next polarizationcombiner 74 combines the split power combinations to produce output 75.One of the combinations is then subject to rotation via quarter waveplate 76 so as to produce output 77. Next, polarization splitter 78separates orthogonal polarizations to produce separated outputs 79.

A simple example explanation of operation of the mixer will now bedescribed. For simplicity of explanation, FIG. 5 and FIG. 6 illustrateexamples of polarization state transformation in the polarization mixer.These illustrations are for the case of binary phase demodulation.Taking initially FIG. 5, the case of Phi=0 degrees is illustrated, theinput polarization 80 is assumed to be vertical. The mixer input 81 isin the horizontal state. The power splitter 73 splits the power of theinput producing polarization outputs 82. The combiner 74 combines thetwo polarization states into their vector sum. In this case the twoinputs are combined ‘vectorially’ to produce vector sums 84, 85. Thequarter wave plate 76 delays one polarization axis relative to the otherso that horizontal polarization state 84 is transformed into circularpolarization state 86. The polarization splitter 78 then providespolarization outputs 88. In this particular instance of Phi=0, where noquadrature signal is encoded and only binary phase demodulation isimplemented, equal power is initially distributed into each top channelso that no distinguishable quadrature signal is output. However, in thebottom two channels, the equivalent of a binary ‘1’ is output with allthe power from the vertical polarization state 85 going into one channeland zero power being output into the other channels.

FIG. 6 illustrates the case when Phi=180 degrees and it can be seen thatsimilar processing occurs through the element 73, 74, 76 and 78.Initially, the polarization states 90, 91 are distributed 92 by element73. Next, they are combined vectorially via polarization combiner 74 toproduce vectorially combined polarization states 94, 95. Thepolarization state 94 is subject to quarter wave plate relative delay toproduce elliptical/circular polarization state 96 with the polarizationstate 95 being unaffected 97. Finally, polarization splitter 78 acts onsignal 97 to produce output signals 98. Again, in this situation noquadrature signal is provided on the upper output ports with the outputpower being distributed equally on both channels. However, in the lowerchannels, the polarization state 97 results in an output ‘1’ in thelower channel which is interpreted as an output equivalent to binary(0).

Where quadrature encoding of the polarization signals is utilized, thetop two channel outputs provide an indicator of the phase encoding ofthe signals.

FIG. 7 illustrates one form of implementation of the preferredembodiment in more detail. FIG. 7 needs to be ideally read inconjunction with FIG. 8 which illustrates a corresponding evolution ofpolarization states for the arrangement of FIG. 7. The transmittedsignal and local oscillator are input on input fibers 101, 102.Referring to the polarization states, the local oscillator is inputhaving an input polarization of 45 degrees to the vertical. The receivedsignal has a randomly oriented polarization state 105 depending on thecurrent transmission alignment and transmission encoding.

The polarization diversity element 73 acts to spatially separateorthogonal polarization components producing polarization outputs 107.The diversity element consists of a first walk off crystal 109, a halfwave plate 110 and a second walk off crystal 111. As is known in theart, the walk off crystals 109, 111 act to spatially separate orthogonalpolarization states. The walk off crystal can be a birefringent YVO₄(Ytrium Vanadate) crystal with the optical axis aligned at approximately45 degrees to the face of the crystal in the direction of the requiredseparation of the polarization states. In this case, the first walk offcrystal 109 acts to translate the horizontally polarized input in avertical manner. The half wave plate 110 rotates both polarizationstates by π/2 degrees. The second walk off plate 111 thereaftertranslates the newly horizontal polarization state downwards. The netresult of this operation, as will be evident to those skilled in theart, is to separate the horizontal and vertical components of the inputsignals to produce polarization outputs 107.

Next, the polarization splitter element 120 acts to power split thespatially separated signals 107 into corresponding polarization outputs121. The splitter consists of two walk off crystals 123, 124. Thecrystals are oriented at +45 degrees and −45 degrees to the horizontalaxis respectfully. The resulting effect of the two walk off plates isillustrated in polarization state diagram 121 which illustrates thateach of the previous polarization states 107 have been rotated anddivided into two components. The polarization state of each componentbeing at +/−45 degrees to the horizontal.

Next, a half wave plate array 127 acts to align the polarization statesinto vertical and horizontal components 126. The walk off plate 129translates the vertical polarization component so that it overlaps withthe horizontal components as indicated 128.

The quarter wave plate 75 is placed centrally to act on the middle twochannels only and acts to delays one polarization component relative tothe other by π/4 radians. This provides for determining the quadraturephase terms and allows an unambiguous determination of the relativephase between the oscillator and signal via analysis of the horizontaland vertical polarization states with a 90 degrees offset in the phase.

The polarization splitter 76 comprises walk off crystals 131, 132aligned at +/−45 degrees to the horizontal respectively. Thepolarization splitter 76 acts to distribute the power of thepolarization states 128 for output on output fiber array 136. Theoutputs include vertical polarization outputs, horizontal polarizationoutputs and 90 degree delay terms.

The embodiment provides for a means for processing polarization andphase encoded information in an optical signal. It is not limited tofiber optical transmission. Indeed, the present invention hasapplication wherever it is desired to decode polarization stateinformation in an input signal.

FIG. 9 illustrates one form of simulated optical train implementing thearrangement of FIG. 2. In this arrangement, the polarization separationis achieved by wedges rather than walk off plates. In this arrangement,the local oscillator signal is input on fibre 140 with the input signalsbeing input on fibre 141. The polarisation diversity and splitting inaddition to the polarisation alignment portions are provided by opticalcomponents 142 which include a series of polarization wedges and halfwave plate array[[ ]]. A lens 143 is also provided for focusing theinput beams. The quarter wave plate 144 acts only on predeterminedportions of the signal train. The polarisation combining is provided bybirefringent wedges 145 and polarization analysis is provided bywalk-off plates 146. The output signals are then reflected onto PiNArray 148 by prism 147.

For some modulation formats (such as ODB, DPSK and DQPSK), a logicalexclusive-OR (or modulo 2 addition) is necessary in the demodulator. Itis possible to implement such a function in the optical domain using anoptical delay line. With an optical delay line, the incoming opticalsignal is split into two paths. The signal in one path is delayed by atime corresponding to one bit and the signals in the two paths arecoherently re-combined.

Thus, if the optical signal and the delayed optical signal are in phase,the sum output will be comparable in magnitude with the original opticalsignal whereas the difference output will be approximately zero. If thesignals are π radians out of phase, the difference output will becomparable in magnitude with the original optical signal whereas the sumoutput will be approximately zero.

Through the utilization of an initial delay line on the front end of thefirst embodiment, the need for a local oscillator can be dispensed with.The resultant overall device is illustrated schematically in FIG. 10. Inthis arrangement, the input signal initially proceeds through an opticaldelay line 150 to produce signal output 151 and delayed output 152, withthe delay output 152 being delayed by one period relative to the output151.

The outputs are fed to the optical processing unit 30 which produces aseries of outputs 153, in the usual manner.

The delay line can take many forms. One form is illustrated in FIG. 11and provides a free space delay line 150. An input signal 161 is fedthough first and second transparent plates 162, 163. The plate 163 has a50% silvered mirror 164 on one surface thereof 50% of the light isoutput as the signal. The reflected light is transmitted to a secondsilvered mirror 165 which provides for 100% reflection. The furtherreflected light is then output as the delayed signal. The distance thedelayed signal has to travel is constructed to be equivalent to theoptical period of the input signal. The delay line 150 is preferablyconstructed from materials having a low coefficient of thermal expansionand operated in a stable temperature environment so that the delay pathlength variation is minimized in the presence of temperature changes.

FIG. 12 illustrates a simulated optical train implementation 170 of theembodiment including a delay line. In this arrangement, the input signalis input 170 into delay line block 171. The delay line block 171implements the delay line 150 of FIG. 11 to produce a signal output anddelay output. Next wedges 73 implement the polarization diversity unitof FIG. 7. This is followed by polarization splitter 120, focusingoptics 172, polarisation alignment 127, 129 and Quarter Wave Plate 75.Additionally polarisation combiner 76 is utlised before the light isreflected via prism 173 onto a PiN array 174 for analysis.

Other modifications are possible. For example, where phase informationonly is to be detected, a polarization independent phase detector couldbe constructed through the substitution of the polarization separatingwedge of FIG. 9 with a polarization walk off plate such that, at thefocal plane of the lens, the orthogonal polarizations coalesce creatingonly 4 spots and polarization independence of the received power.

The embodiments can be utilized in many different forms for themonitoring of incoming signals. Where optical telecommunications areutilized, the information can be decoded. The embodiments can also beutilized in the measurement of signals derived from sampling of imagingsystems or metrology systems.

Interpretation

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in one embodiment” or “in an embodiment” in various places throughoutthis specification are not necessarily all referring to the sameembodiment, but may. Furthermore, the particular features, structures orcharacteristics may be combined in any suitable manner, as would beapparent to one of ordinary skill in the art from this disclosure, inone or more embodiments.

Similarly it should be appreciated that in the above description ofexemplary embodiments of the invention, various features of theinvention are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureand aiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the Detailed Description are hereby expressly incorporatedinto this Detailed Description, with each claim standing on its own as aseparate embodiment of this invention.

Furthermore, while some embodiments described herein include some butnot other features included in other embodiments, combinations offeatures of different embodiments are meant to be within the scope ofthe invention, and form different embodiments, as would be understood bythose in the art. For example, in the following claims, any of theclaimed embodiments can be used in any combination.

Furthermore, some of the embodiments are described herein as a method orcombination of elements of a method that can be implemented by aprocessor of a computer system or by other means of carrying out thefunction. Thus, a processor with the necessary instructions for carryingout such a method or element of a method forms a means for carrying outthe method or element of a method. Furthermore, an element describedherein of an apparatus embodiment is an example of a means for carryingout the function performed by the element for the purpose of carryingout the invention.

In the description provided herein, numerous specific details are setforth. However, it is understood that embodiments of the invention maybe practiced without these specific details. In other instances,well-known methods, structures and techniques have not been shown indetail in order not to obscure an understanding of this description.

As used herein, unless otherwise specified the use of the ordinaladjectives “first”, “second”, “third”, etc., to describe a commonobject, merely indicate that different instances of like objects arebeing referred to, and are not intended to imply that the objects sodescribed must be in a given sequence, either temporally, spatially, inranking, or in any other manner.

In the claims below and the description herein, any one of the termscomprising, comprised of or which comprises is an open term that meansincluding at least the elements/features that follow, but not excludingothers. Thus, the term comprising, when used in the claims, should notbe interpreted as being limitative to the means or elements or stepslisted thereafter. For example, the scope of the expression a devicecomprising A and B should not be limited to devices consisting only ofelements A and B. Any one of the terms including or which includes orthat includes as used herein is also an open term that also meansincluding at least the elements/features that follow the term, but notexcluding others. Thus, including is synonymous with and meanscomprising.

Similarly, it is to be noticed that the term coupled, when used in theclaims, should not be interpreted as being limitative to directconnections only. The terms “coupled” and “connected,” along with theirderivatives, may be used. It should be understood that these terms arenot intended as synonyms for each other. Thus, the scope of theexpression a device A coupled to a device B should not be limited todevices or systems wherein an output of device A is directly connectedto an input of device B. It means that there exists a path between anoutput of A and an input of B which may be a path including otherdevices or means. “Coupled” may mean that two or more elements areeither in direct physical or electrical contact, or that two or moreelements are not in direct contact with each other but yet stillco-operate or interact with each other.

Although the present invention has been described with particularreference to certain preferred embodiments thereof, variations andmodifications of the present invention can be effected within the spiritand scope of the following claims.

1-15. (canceled)
 16. A method of analysing an input optical signal, themethod including the steps of: (a) dividing a the input signal intofirst and second orthogonal signal polarisation components; (b) dividinga local oscillator signal having at least a first frequency into firstand second orthogonal local polarisation components; (c) polarisationmixing the first signal polarisation component with the second localpolarisation component to provide a first mixed signal, wherein thefirst signal polarisation component is orthogonal to the second localpolarisation component; (d) polarisation mixing the second signalcomponent with the first local polarisation component to provide asecond mixed signal, wherein the second signal polarisation component isorthogonal to the first local polarisation component; and (e) analysingthe first and second mixed signals to determine optical powerinformation of the first and second mixed signals at at least the firstfrequency.
 17. A method as claimed in claim 16 wherein said input signalincludes information encoded in a periodic signal.
 18. A method asclaimed in claim 16 wherein step (e) further includes the steps of: (e1)splitting the power of the first mixed signal into at least a first andsecond mixed sub-signals; (e2) delaying one orthogonal polarisationcomponent of the first sub-signal relative to the second component bypi/2 radians to produce a phase delayed first sub-signal; (e3) dividingthe phase delayed first sub signal into orthogonal components and thesecond mixed sub-signal into orthogonal components.
 19. A method asclaimed in claim 16 wherein said step (e) further includes the steps of:(e1) delaying one orthogonal polarisation component of the firstsub-signal relative to the second component by pi/2 radians to produce aphase delayed first sub-signal; (e2) dividing the phase delayed firstsub signal into orthogonal components and the second mixed sub-signalinto orthogonal components.
 20. A method of analysing an input opticalsignal as claimed in claim 1 wherein, in step (d), phase information ofeach of the first and second orthogonal signal polarisation componentsis determined at the first frequency.
 21. An apparatus for analysing aninput signal, the apparatus including: a polarisation diversity elementinterconnected to said input signal and a local oscillator signal havinga first frequency for separating the optical power of the input signalinto first and second orthogonal signal polarisation components and forseparating the optical power of the oscillator signal into first andsecond orthogonal local polarisation components; a first polarisationsplitting element for further splitting the optical power of the firstand second orthogonal signal polarisation components into furtherorthogonal subcomponents; a polarisation element for polarisationaligning predetermined ones of said subcomponents; a polarisationtranslation element for spatially aligning groups of polarisationaligned sub-components to produce a series of spatially alignedsubcomponents; and a second polarisation splitting element for splittingthe optical power of the aligned subcomponents into output componentshaving optical powers at at least the first frequency.
 22. An apparatusfor decoding polarization encoded input signals, the apparatusincluding: a local oscillator outputting a local oscillating signalhaving a predetermined polarization state and a first frequency; aninput for inputting said polarization encoded input signal; apolarisation diversity element interconnected to said input signal andlocal oscillating signal for separating the optical power of the inputsignal into first and second orthogonal signal polarisation componentsand for separating the optical power of the oscillator signal into firstand second orthogonal local polarisation components; a firstpolarisation splitting element for further splitting the optical powerof the first and second orthogonal signal polarisation components intofurther orthogonal subcomponents, and first and second orthogonal localpolarisation components into further orthogonal subcomponents; apolarisation element for polarisation aligning predetermined ones ofsaid sub-components; a polarisation translation element for spatiallyaligning groups of polarisation aligned sub-components to produce aseries of spatially aligned subcomponents; and a second polarisationsplitting element for splitting the optical power of the alignedsubcomponents into output components having optical powers at at leastthe first frequency.